ACTIVE REDUNDANCY CONCEPT PROVEN BY OPERATION EXPERIENCE ON PFEIFFER MVR TYPE VERTICAL ROLLER MILLS Bernd Henrich, Area Manager*, Falk Werner, Senior Sales Manager* Dr. Dirk Hoffmann, Head of Development Group* * Gebr. Pfeiffer SE, Kaiserslautern, Germany Germany
1.
Introduction
In the year 2010 the MVR mill for grinding large and very large production rates was presented on the market by Gebr. Pfeiffer SE for the first time. The innovation of the mill concept lies in the active redundancy of the grinding rollers and of the drive which has been attracting much interest from cement customers and technical consultants all over the world. The special feature of this concept is the option of continuing the grinding process in the MVR mill even with one or two grinding rollers being out of service due to a scheduled maintenance shutdown or an unplanned incident. For example, if maintenance work has to be done on one of the rollers of a six-roller MVR mill with MultiDrive®, this roller will be swung out of the mill, the maintenance door will be closed again and mill operation can be continued. The production rate will be somewhat reduced though, but there will only be a short plant shutdown which is most important to the customer. Up to now more than 20 MVR mills were sold to customers all over the world. The customers’ operating experiences prove that the concept works very reliably. In fact when operating in part-load operation, i.e. with a reduced number of rollers, the mills achieve higher production rates than expected. Together with a customer, operation of a cement mill with a reduced number of drives was tested as well.
2.
MVR vertical roller mill technology
Kiln capacities in the cement industry have now reached clinker production rates of up to 12,000 t/d. Figure 1: MVR vertical roller mill This requires mills for the production of up to 1,000 t/h
raw meal. Lowest investment costs are managed with single mill solutions. This implementation is inevitably linked with the requirement for increased plant reliability, availability and ease of maintenance. The MVR vertical roller mill in conjunction with the patented MultiDrive® systems [1, 2] fulfills these requirements. By introduction of the MVR mill with a roller module system and a new drive system, the active redundancy concept provides the highest availability of the plant. Figure 1 shows the 3D sectional view of an MVR mill with six rollers. The geometry of grinding parts is illustrated in the 3D view in Figure 2. Loading and preparation for shipment is demonstrated in Figure 3 for one of the four rollers of the MVR 5600 C-4. The weight of one roller unit with wear parts is about 46 tons. The new MVR roller mill is characterized by four or six grinding rollers and the use of flat grinding table liners. A roller unit consists of the roller with cylindrical roller tyre, roller axle, roller arm, support and the hydraulic tension system. In conjunction with the flat grinding table liner geometry, that type of roller suspension system achieves a
Figure 2: Geometry of grinding parts
parallel grinding gap between roller and table liner at any time. This results in a positive effect on the vibration level of the mill and the energy input into the Figure 3: Lifting a roller (of MVR 5600 C-4 mill) for truck loading grinding bed. In most cases two adjacent roller units are connected to the foundation through a twin support. This gives benefits on accessibility and improves the arrangement of hot gas ducts, the implementation of the external material recirculation and the installation of auxiliary equipment. The roller units can be swung out individually for maintenance purposes using the same hydraulic system that applies the grinding force during operation. The machine parts relevant in terms of fluid dynamics, such as hot gas channel, nozzle ring, SLS highefficiency classifier and material feed have the same design as the parts that have proved and tested successfully in the well-known MPS mills of Gebr. Pfeiffer SE. With the new MultiDrive® gearbox and table thrust bearing, the grinding table is driven through a girth gear by up to six identical drive units. Each unit consists of an electric motor, coupling and bevel/spur gear unit arranged on a base frame with slide rails. The grinding forces are transmitted to the foundation via a conventional plain bearing without placing any loads on the gear units.
With the MultiDrive® and its several drive units, operation can be continued after lifting or swinging out only one roller. Due to this active redundancy concept, the MVR roller mill equipped with the MultiDrive® will produce about 85% of the rated capacity even when maintenance is needed on a roller or the drive system. [3] When the table is driven through a planetary gear unit instead, production can only be maintained at a reduced throughput of about 60% theoretically after two opposing rollers have been swung out or lifted up.
3.
Case study Balaji plant: n-1 rollers in operation
The cement mill type MVR 5600 C-4 was put into operation in May 2012. This mill is equipped with four grinding rollers and has a MultiDrive® with four drive modules of 1,650 kW nominal drive power each. Now that the mill has been operating for about 3 years with no need at any point in time to run with a reduced number of grinding rollers, the technical staff of the final customer Jaiprakash Ass. Ltd. asked Gebr. Pfeiffer to do test operation with n-1 rollers. As this test operation could be well planned without any time pressure, quite a number of
Figure 4: Strap possitioned to secure the roller out or operation
data were collected which are not analyzed during normal operation. A commissioning specialist from Gebr. Pfeiffer (India) Pvt. Ltd was sent to the site to support the operating staff of Jaiprakash Ass. Ltd. during the investigations. Moreover, specialists from Gebr. Pfeiffer SE of Kaiserslautern joined them to examine the effects of the n-1 operation on mechanical components, process aspects, and electric parts. Specialists from Siemens, supplier of the electromechanical drive system, took also part in the investigations. First of all, the so-called operation mode “zero“ was determined, with the mill producing standard OPC with a product fineness of about 3,000 cm²/g Blaine and a production rate of about 290 t/h. As the kiln was off line at that point of time, this was the typical operation mode of a grinding plant, i.e. cement grinding in the mill without any external heat. Figure 5: strap in stand by possition
In the next step, one grinding roller was taken out of service. For this purpose the grinding plant was stopped. The roller was lifted with a mobile unit for local control and with the hydraulic tension system comprising tension cylinder, oil tank, high pressure pump etc., which is also used during normal plant operation. To simulate operation mode n-1, it was not necessary to swing the roller out of the mill. For safety reasons, the lifted roller has to be secured with a strap in a way to avoid that the roller moves back down onto the grinding bed (see Figure 4). Figure 5 shows the roller with the strap loosened, i.e. in normal operation mode. In Figure 6, the oil hoses of the mobile unit, which are connected to the tension cylinder, can be seen. With one roller less, the grinding plant was put into operation again. Table 1 gives the most important operating data. It was proven that the grinding plant operating with only 75% of the total number of grinding rollers Figure 6: Mobile control unit connected by hoses produced roughly 83% of the normal production rate.
CEM I (96/4)
Zero test
n-1
Product rate
291 t/h
240 t/h (83%)
Fineness
3,000 Blaine
Table seed
88%
!ec" o#er cons"
13"0 $h/t
1&"1 $h/t
n'otor
100 0"8%
100 1"8%
Torue *F+
100 10%
100 23%
Table 1: Operating data MVR 5600 C-4 in n-1 operation mode
Under processrelated aspects, operation had remained almost unchanged as compared to normal operation with the full number of grinding rollers. To ensure the target fineness of the cement, classifier speed was maintained nearly constant. To adapt to the smaller product rate to be carried out of the classifier into the filter, the volume flow of the plant fan was slightly reduced so that dust load
after classifier remained practically the same. The pressure of the hydropneumatic tension system remained unchanged. The filling degree of the recirculating bucket elevator and its power consumption showed that the amount of recirculating material had not increased significantly. This is most probably due to the fact that the volume flow through the system was not reduced in the same degree as the production rate went down. It was found that absorbed power of the drive units went down whereas the variation of current consumption (around the average) slightly went up. Such variation, which is just below 1% in normal operation, was nearly double (1.8%) but still acceptable. On the shafts of the drive units, torque measurements were done. It was found that torque dynamics also doubled to ± 23% which is, however, still uncritical for the drive unit. It can be stated as a result that mill operation with only 3 rollers was as smooth and trouble free as normal operation with 4 rollers. It was demonstrated to the customer that he could switch to n-1 operation without any concern.
Figure 7 summarizes the points which are most important under mechanical aspects. Due to the non-uniform load exerted by the 3 grinding rollers onto the grinding bed and transferred through the grinding bowl into the table thrust bearing, the grinding bowl starts wobbling, i.e. it is on a slightly higher level in the position of the lifted roller. The measured difference was 0.6 mm. In the
positions of the two other rollers (staggered 90° apart), the difference was about 0.3 mm each. These values were as expected. During the grinding process, there are not only vertical grinding forces but also tangential forces exerted by the rollers onto the grinding bed and grinding bowl. With an asymmetric number of rollers in operation, forces are created in the table thrust bearing which strive to move the latter out of its central position. The MultiDrive® with its integrated radial bearing system is designed to take up such forces. During operation with n-1 rollers, the radial displacement of the girth gear was measured to be about 0.3 mm, hence in a range as expected. Such girth gear displacement does not pose any problem to the drive. In fact the individual gear units are connected with the girth gear via self-aligning pinions and may therefore adapt automatically to a change of the contact situation (mesh).
4.
Scheduled maintenance at Ras plant of Shree Cement: operation with 4 rollers against 6
In the Indian cement works of Shree Cement Ltd. in Ras/Rajasthan, a large number of MPS mills for the grinding of cement raw material and coal/pet coke are installed along with several mills type MVR 6000 C-6 for cement grinding. This mill type, currently one of Gebr. Pfeiffer’s best-selling mills, has 6 grinding rollers. For a drive power requirement of about 6,000 kW, the customer decided to equip the mills with conventional bevel planetary gearboxes. Such gearboxes are not designed to take up asymmetric loads from the grinding process and/or increased radial loads. However, the customer may profit from the active redundancy concept of the rollers. If one roller has to be swung out of the mill, the opposite roller must be taken out of service as well. But it just needs to be lifted and secured by mechanical means as described above. In the Ras plant, damage had occurred on a sealing of a roller. During scheduled maintenance work, the customer’s staff was able within a short time to swing the roller concerned out of the mill and to lift the opposite roller out of the grinding bed. The staff managed to put the mill back into part-load operation within less than one shift’s time. After replacing the sealing outside the mill, hence in a safe working area, the roller was swung back into the mill later on so that the mill could be operated with 6 active rollers again.
5.
Holcim analysis of n-1 drive units operation at Val de Seine plant
Apart from Lafarge who ordered 4 MVR mills so far, Holcim also ranges among our main customers with several cement mills including the biggest vertical mill worldwide, type MVR 6700 C-6 installed in Brazil for cement grinding with a nominal power of the drives of a total 11,500 kW. The concept of the MultiDrive® where the drive torque is distributed onto a number of 2 to 6 drive units was realized for the first time on an MPS mill in the Val de Seine works in France. In view of the good operating results, Holcim selected this drive system for the MVR 6000 C-6 installed in Australia and also for the above mentioned MVR 6700 C-6 in Brazil. Apart from profiting from active redundancy during mill operation, the use of identical drive units is of particular importance with regard to spare parts stock keeping.
The above 3 cement mills have a total 12 identical drive units. The customer decided to put one drive unit into stock so that whenever a problem would occur on one of the mills and a gearbox would have to be replaced, the spare unit could be taken to the works within a short time. To be noted that the transport weight is only about 25 tons, hence much lower than that of complete conventional drives for vertical mills. Until the spare unit arrives, the customer continues running the mill and hence the grinding plant thanks to the active redundancy principle of the drives, with a slightly reduced production rate though. It has not been necessary so far to take one drive unit of the MultiDrive ® out of service. This situation was, however, tested by Holcim together with Gebr. Pfeiffer using the example of the mill installed in France. On this occasion, extensive measurements along with analyses of the recorded data were made.
Figure 8: Basic data of MultiDrive® on MPS 4750 B in France First of all, the so-called operation mode “zero“ was determined, with all three drives of the mill being in operation and the mill producing cement of the type CEM I. In the next step, one drive unit was taken out of service. For this purpose, the entire unit comprising motor, coupling, and gearbox, which are installed on a common carriage, can be removed radially from the girth gear. Thereupon the opening in the girth gear housing provided for this purpose was closed and the mill was restarted. Although 33% of the drive power were not available, it was possible to achieve roughly 75% of the production rate as compared to the quantity produced with all drives. Absorbed power went down as expected. The dynamic effects regarding the torques of the drive units did not show any major changes as compared to operation mode “zero”. The measured vibration velocity, which is one of the indicators of smooth run of the mill, increased slightly but remained on an acceptable level. The analysis of the force situation in the system with 3 drives as shown in Figure 10 reveals that the essential drive forces are exerted tangentially on the common girth gear. In addition there is a radial force component resulting from the contact/mesh of pinions and girth gear. If one drive is out of mesh during n-1 operation, there is an imbalance of forces. This results in a radial load on the table thrust bearing. The MultiDrive® is equipped with a hydrostatic radial bearing that takes up these forces, thus allowing n-1 operation.
Figure 9: Operating date of MPS 4750 BC in France with n-1 drives
Figure 10: Situation at drives of MPS 4750 BC in France with n-1 drives
The measurements done on the mill in France show that there was a displacement of the girth gear by about 0.2 mm. This rate is well within the range expected and even significantly below the value tolerated by the system.
6.
Final remarks
In connection with the investigations regarding n-1 operation (grinding rollers and drives), extensive measurements of process data were carried out. The drive system was also analyzed in detail. It was evidenced that the expectations were fully met and even exceeded. The active redundancy concept of the grinding rollers is beneficial to Pfeiffer mills of the type MVR no matter whether these are equipped with a MultiDrive® or a conventional bevel planetary gearbox. But with the installation of a MultiDrive® on the MVR mill, the customer additionally profits from the advantages of active redundancy regarding the drive system. The customer operating very large mills with drive powers of plus 12,000 kW will feel at ease knowing that his plant will continue running even if an unscheduled stop occurs on a drive unit or when there is a scheduled maintenance stop on a drive unit. Conventional bevel planetary gearboxes are available up to power ratings of about 7,000 kW. Currently studies are accessible on the market, presenting various drive systems for vertical mills with installed powers of more than 7,000 kW. These drive systems were developed by gearbox and/or motor manufacturers, some with several motors, but none with several drives. Just a few of such concepts are being analyzed on pilot plants but long-term experience is not available for any of them. In contrast the MultiDrive® and active redundancy roller system developed by Gebr. Pfeiffer have a decisive advantage: this drive system has been running in industrial operation for several years a lready to the full satisfaction of the final customers. Any start-up problems that may occur on every innovation have been remedied. The MVR mill with its active redundancy concept of grinding rollers and its MultiDrive ® has passed through extensive research and development for about 10 years with more than 5 years of operation experience, offering each customer a system which is innovative but also proven.
7.
References
[1] EP 2252403: Roller Grinding Mill [2] EP 2086685: Safety System for Roller Mill
